Preferred ADI/R: a permanent pacing mode to eliminate ventricular pacing while maintaining backup support

ABSTRACT

An ADI/R mode is implemented using an intelligent pacing system to continually monitor ventricular response. This ensures AV conduction whenever possible so as to gain all the benefits of cardiac contractile properties resulting from native R-waves. In the event where AV conduction is blocked, the pacing mode is switched to a DDD/R mode to ensure a paced R-wave. Thereafter, subsequent to a completed interval of a p-wave, ADI/R pacing resumes to monitor ventricular response.

FIELD OF THE INVENTION

[0001] The present invention generally relates to cardiac pacers, andmore particularly to a dual chamber rate responsive pacemaker thatincorporates a novel ADI/R pacing mode. More particularly, this pacingmode is one that gives preference to atrial pacing and, at the sametime, suppresses ventricular pacing wherever possible and provides theDDI/R or DDD/R modes with ventricular pacing only as backup modes.

BACKGROUND OF THE INVENTION

[0002] Early pacemakers were asynchronous (VOO) and stimulated the heartat a fixed rate, independent of the patient's underlying cardiac rhythmor metabolic demand. Although such pacers, typified by U.S. Pat. No.3,057,356 to Greatbatch, provide a ventricular pacing rate sufficient tosustain life, this pacing mode often competed with native ventricularrhythms. Such competition is undesirable.

[0003] Subsequently, demand pacemakers (VVI) were developed. This typeof pacer interacts with the patient's heart to provide pacing pulsesonly when spontaneous ventricular activity is absent. U.S. Pat No.3,478,746 to Greatbatch demonstrates an example of such a pacer. Thisform of pacer provides a ventricular sense amplifier for detectingventricular depolarizations. A ventricular sensed event resets thepacer's V-V timer. The ventricular sensed event also cancels or inhibitsthe scheduled ventricular stimulus and thus avoids competition with thenative ventricular rhythm.

[0004] Atrial synchronized pacers (VAT) were developed almostsimultaneously with VVI demand pacemakers. This type of pacer paces theventricle in response to the detected atrial rate of the patient. TheVAT pacer, as typified by U.S. Pat. No. 3,253,596 to Keller, provides anatrial sense amplifier for detecting atrial depolarizations. An atrialsensed event starts the pacer's A-V delay timer. When the A-V timertimes out, a ventricular stimulus is provided. Conceptually, such apacer can be considered as a prosthetic conduction pathway thatsimulates the natural A-V conduction pathways of the heart. One drawbackto this form of pacing is the possibility of competing with ectopicventricular activity. An ectopic ventricular beat (PVC) may be detectedin the atrium. In such cases, an AV interval starts and will result inthe generation of a ventricular stimulus a short time after theventricular depolarization. Although such a pacing regimen is consideredharmless when the A-V delay is short, it is possible to deliver thepacing stimulus into the vulnerable period of the ventricle, and therebyinitiate a ventricular arrhythmia.

[0005] Continued development of pacemakers was marked by the inventionof the AV sequential pacer (DVI), as disclosed in U.S. Pat. No.3,595,242 issued to Berkovits. This form of pacer provides forstimulation in both the atria and the ventricles though providingsensing only in the ventricle. In this DVI mode pacer, a ventricularsense event starts both a V-A escape interval and an A-V interval. Thepacer delivers an atrial stimulus at the end of the V-A interval and, atthe end of the A-V interval, the pacer delivers a ventricular stimulus.If a ventricular sense event occurs during the V-A or A-V timeintervals, the pacer will resynchronize to the ventricular sense eventand inhibit the delivery of the scheduled ventricular stimulus.

[0006] The DDI mode pacer described by U.S. Pat. No. 3,747,604 toBerkovits further includes an atrial sense amplifier to inhibit theatrial stimulus when an atrial sense event occurs during the V-Ainterval. The atrial sense event does not start and A-V interval; suchtiming makes this device especially suitable in patients where atrialcompetition must be avoided.

[0007] The atrial synchronized ventricular inhibited or VDD mode pacer,as disclosed in U.S. Pat. No. 3,648,707 issued to Greatbatch hasmechanisms for sensing in the atrium and ventricle while providingstimulating pulses only in the ventricle. In operation, the VDD pacerstarts and A-V interval on detected atrial activity and provides aventricular stimulus if one does not occur within the A-V delay. Aventricular sensed event inhibits the scheduled ventricular stimulus andresets the pacer's V-V timer.

[0008] The dual sense, dual pace DDD mode pacers, have been described inU.S. Pat. No. 4,312,355 issued to Funke. The DDD pacer addresses many ofthe shortcomings of the prior art devices. The DDD mode pacer, asdescribed by Funke, has had wide applications. This type of pacer hassense amplifiers for detecting atrial and ventricular events, as well asoutput pulse circuitry for stimulating both the atrium and theventricle.

[0009] This form of prior art pacer provides timing circuitry toinitiate an A-V delay upon the occurrence of an atrial event. If, duringthe A-V delay period, no spontaneous ventricular event is sensed, thepacer will produce a ventricular stimulus at the conclusion of the A-Vdelay. If, during the V-A interval, no spontaneous atrial event issensed, the pacer provides an atrial stimulus at the conclusion of theV-A interval.

[0010] In this type of pacemaker, in the absence of spontaneous P-wavesand R-waves, the heart will be stimulated at fixed A-A and V-V intervalswith a programmable AV delay. However, if the ventricle depolarizesspontaneously, then the A-V delay is truncated and the observed A-Ainterval is not fixed and will be shorter than the arithmetic sum of theprogrammed A-V and V-A intervals.

[0011] The dual chamber modalities, DVI, VAT, VDD and DDD, have provento be especially efficacious pacemakers since they restore A-V synchronyand thus improve cardiac output by ensuring the hemodynamic contributionof the atrial chambers within the pacing regimen. The latter three modesalso synchronize the pacing rate to the patient's native atrial or sinusrate and thus provide an increased pacing rate in response to bodilyactivity. Increasing cardiac rate is the major contributor to increasedcardiac output. More recently, other pacers, which increase cardiacoutput in response to exercise, have been proposed. They includepacemakers that rely upon the sensing of physical via an activity sensoror accelerometer, changes in blood pH, respiratory rate, or QT interval.These data are used to alter the pacemaker's escape interval.

[0012] One approach that is important to the understanding of thepresent invention is the activity responsive pacer described in U.S.Pat. No. 4,428,378, issued to Anderson et al, and which is incorporatedby reference. The pacer disclosed in that patent monitors the physicalactivity of the patient and increases the pacing rate in response toincreasing patient activity.

[0013] Other publications that provide background information for theoperation of the present invention include U.S. Pat. No. 4,890,617issued to Markowitz et al. that is incorporated herein by reference.This patent describes a dual chamber activity responsive pacemaker thatsenses and paces in both the atrium and the ventricle. The pacing rateis determined by the sensed activity of the patient, the programmedlower rate, and the patient's atrial or sinus rate.

[0014] U.S. Pat. No. 4,932,046, entitled “Dual Chamber Rate ResponsivePacemaker”, assigned to Medtronic, Inc. of Minneapolis, Minn., which isincorporated herein by reference, describes a dual chamber rateresponsive pacemaker. The pacemaker operates in an atrial-synchronizedmodality when the sensed atrial rate is present, and paces at thesensor-determined rate when the sensed atrial rate is absent or belowthe programmed lower rate.

[0015] The above pacing modes may, in a certain sense, be considered assubsets to the DDD/R mode though, in reality, they were all developedfrom the VVI mode in one way or another. All such possibilities havebeen described in The NBG Code, a five-position code, published andupdated as a joint effort of the North American Society of Pace andElectrophysiology (NASPE) and the British Pacing and ElectrophysiologyGroup (BPEG). This code is generally used by those familiar with thestate of the art and may be found in publications too numerous tomention.

[0016] DDD pacemakers are often implanted in patients with Sick SinusSyndrome (SSS), a term that covers a large array of sinus node diseasestates. Such patients often have intact AV conduction and, if thepacemaker's AV interval is not properly programmed, the pacemaker willdeliver an unneeded and undesirable ventricular pacing pulse. Manypatients who receive DDD pacers or dual-chamber PCD(Pacer/Cardioverter/Defibrillator) devices are unnecessarily paced inthe ventricle. There appears to be reluctance in the medical communityagainst implanting a DDD device and programming it to the AAI/R mode inpatients with sick sinus syndrome (SSS) and intact AV conduction.Moreover, when programmed to the DDD mode, the AV intervals in thesepacemakers may be left at their factory-programmed state, that is, withshorter durations more suitable to third degree AV block patients. Or,even when programmed to a slightly longer duration, the A-V duration maybecome a compromise between a duration that promotes ventricularconduction and one which allows ventricular tracking up to high rates.As a result, ventricular pacing occurs at the termination of theseintervals, with little or no possibility of spontaneous ventricularactivity being allowed.

[0017] There is growing medical evidence that inappropriate ventricularpacing has disadvantageous short-term hemodynamic effects and may proveharmful when allowed to continue for an extended period of time. It hasbeen know in the art as early as 1925 that ventricular pacing results inasynchronous delayed activation of the ventricular tissue and, thereby,produces compromised hemodynamics in mammals. More recently, caninestudies have shown that right ventricular apical (RVA) pacing causes anegative inotropic effect” and a >30% reduction in cardiac efficiency.In addition, long term RVA pacing has been shown to lead to permanentchanges including myofibrillar cellular disarray, myocardial perfusiondefects, and structural abnormalities. Each of these may furthercontribute to deterioration of left ventricular function.

[0018] The various manufacturers, including Medtronic, Inc., haveattempted to address this problem by implementing algorithms thatautomatically adapt the AV interval duration to preferentially allow AVconduction when present.

[0019] In the U.S. Pat. No. 5,861,007, issued to Hess, et al, a SearchAV operation is described in which the pacemaker continuously monitorsfor the presence or absence of an intrinsic R-wave after both sensed andpaced P-waves. The programmed AV interval may be extended by aprogrammable “hysteresis” interval to promote ventricular conduction.The AV interval, however, cannot exceed 350 milliseconds in duration. Tomaintain unimpeded upper rate operation, Search AV works in conjunctionwith Auto-PVARP to maintain atrial sensing and tracking up to theprogrammed upper rate, thereby postponing a 2:1 block operation as longas possible. Since there is a limit to the shortening of the PVARP inthis operation, it becomes necessary to shorten the AV interval afterthe PVARP reaches its maximum decrementation. Consequently, manypatients (>30%) with intact AV conduction are ventricularly paced to asignificant degree (>50%) in spite of having Search AV programmed on.

[0020] Another approach to the problem is presented in U.S. Pat. No.5,318,594 issued to Limousin, et al. The DDD Automatic Mode Switch (AMS)mode operates in a “Special AAI” mode as long as R-wave sensing occurswithin a ventricular surveillance window that is calculated based on thehistory of the measured PR interval. If an R-wave is not sensed withinthis window, the pacing operation switches to the DDD mode. After 100consecutive paced ventricular events, the pacemaker attempts to switchback to the Special AAI mode. Although this operation has been shown toreduce ventricular pacing, because of operational restrictions, it hasbeen only partially effective. A recent study of patients withpredominantly intact AV conduction demonstrates ventricular pacingreduction from a mean of ˜65% to ˜36%.

[0021] A third approach presented in U.S. Pat. No. 6,122,546 issued toSholder et al implements a form of AV/PV hysteresis. This operationencourages intrinsic conduction by extending the AV interval by apredetermined period beyond the programmed duration. As indicated above,this operation is restricted to avoid interaction with upper ratetracking. There is nothing in the literature to indicate one way or theother if it provides a true benefit to the patient. One can assume,however, that the reduction in ventricular pacing will be approximatelythat which has already been cited above.

[0022] Although present in bradycardia pacemakers, AV extensionalgorithms have been absent in dual chamber (DC) cardioverterdefibrillators (ICDs). AV extension presents a unique challenge in DCICDs due to the added requirements of tachyarrhythmia detection. Forexample, to adequately detect a ventricular tachycardia, the AV delaymust be restricted so that the tachy detection interval (TDI) fallswithin the VA interval at all times. Failure to do so comes at theexpense of tachyarrhythmia detection sensitivity. An alternative meansto address this issue is by means of a temporary mode change for aprogrammed period of time following the delivery of a shock.Unfortunately, while this may protect against transient post-shock AVblock, it does so at the expense of beat-to-beat monitoring.Consequently, many electrophysiologists do not program the AAI/R mode ona permanent basis to avoid persistent ventricular pacing.“Ideoventricular kick,” first described by Schlant in 1966,(Circulation, 1966; 23 & 24 (Suppl. III): 209) results from improvedcoherence of the ventricular contraction during normal activation. Thishemodynamic benefit is lost during ventricular pacing. In an earlierstudy of the atrial contribution to ventricular filling (Kosowski B, etal. Re-evaluation of the atrial contribution to ventricular filling:Study showing his-bundle pacing. Am J Cardiol, 1968; 21 518-24), it wasdemonstrated that ventricular function was better during normalventricular activation independent of the PR interval. Similarly, alater study (Rosenqvist M, et al. Relative importance of activationsequence compared to atrioventricular filling synchrony in leftventricular function. Am J Cardiol, 1991; 67(2): 148-56) showed that AAIpacing was superior to either VVI or DDD pacing.

[0023] Aside from the hemodynamic benefits mentioned above, it may bethat normal ventricular activation has a role in preventingtachyarrhythmias. In a study of 77 ICD patients with a mean follow-up of18.7 months (Roelke M, et al. Ventricular pacing induced ventriculartachycardia in patients with implantable cardioverter defibrillators.PACE, 1995; 18(3): 486-91), appropriately timed ventricular pacingpreceded tachyarrhythmia onset in 8.3% of the episodes in five patients.A further study (Belk P, et al. Does ventricular pacing predispose toventricular tachycardia? Abstract. PACE, April, 2000) demonstrates thathigh rate ventricular pacing renders patients more susceptible to theinduction of ventricular tachycardia compared to high rate atrial pacingwith normal ventricular activation.

[0024] These studies, combined with the growing body of evidence showingthe detrimental effects of long-term ventricular pacing, has led to moredeliberate efforts by clinicians to allow for normal ventricularactivation when programming dual chamber bradycardia devices. Still, dueto the interactions imposed by PVARP and upper rate timing, modeswitching, and tachyarrhythmia detection, their best intentions areoften thwarted. The present invention, however, goes a long way towardanswering all the issues posed by previous patents, as well as those inthe published literature.

SUMMARY OF THE INVENTION

[0025] The present invention encompasses a novel mode of a pacing calledthe Preferred ADI/R mode. This mode is intended to be an ON/OFFselection that operates as a subset of the programmed DDD/R mode.Although this mode is framed in the standard NBG nomenclature, it isdoes not use the “bottom-up” approach (that is, stemming from the VVImode), but rather the “top-down” approach (that is, deriving from an A-Vperspective). This mode is primarily indicated for use with SSS patientswho constitute some 66% of all patients who received a pacemaker. Someof these patients have concomitant third degree AV block. There remains,however, a significant majority who either have intact AV conduction, orAV block in which AV conduction is present in varying degrees (that is,first degree or second degree Mobitz type I).

[0026] When programmed to the ADI/R mode, the pacemaker checks on abeat-to-beat basis for intact AV conduction. The pacemaker will continueto pace the atrium and allow the conducted ventricular event to takeplace. If, however, intermittent AV block does occur, the modeautomatically switches to the DDI/R or DDD/R mode for one, or as manycycles as necessary, and upon the detection of the presence of AVconduction, the mode switches back to the ADIR/R mode.

[0027] The preferred ADI/R mode operates preferentially as itcontinually monitors the ventricular response. This mode may be used inthose patients with intact AV conduction or intermittent AV block. Thepurpose of this mode is to ensure AV conduction whenever possible so asto gain all the benefits from the contractile properties accruing fromnative R-waves. In those instances where the AV conduction system isintermittently blocked (such as might occur in a rate-induced block),the pacing mode is automatically switched to a committed DDI/R mode witha short AV delay to ensure a paced R-wave. The cycle that follows is inthe DDD/R mode with a longer AV delay to determine if conduction hasreturned. If so, ADIR pacing resumes with continued monitoring forintrinsic AV conduction.

[0028] The present invention, that is, the permanent pacing mode,Preferred ADI/R, is indicated for patients with sick sinus syndrome,with intact AV conduction, first degree or second degree (Mobitz II)block. This mode is superior to permanent DDD/R, especially in ICDpatients. This mode allows for greater programming flexibility and fewerinteractions with other programmable parameters. Intrinsic conductionand normal ventricular activation/contraction is promoted by this modewith all of its attendant benefits. In addition the DDD/R mode isavailable as a backup mode providing a safety net for those times when apatient unexpectedly experiences intermittent high grade AV block.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029]FIG. 1 is an illustration of a body-implantable device system inaccordance with the present invention, including a hermetically sealeddevice implanted in a patient and an external programming unit.

[0030]FIG. 2 is a perspective view of the external programming unit ofFIG. 1.

[0031]FIG. 3 is a block diagram of the implanted device from FIG. 1.

[0032]FIG. 4 is a ladder diagram of the ADI/R operation.

[0033]FIG. 5 is a ladder diagram of the committed DDD/R operation in theevent that the patient develops transient AV block.

[0034]FIG. 6 is a ladder diagram that depicts the pacing operation inthe event that the patient develops AV block that persists for more thanone cycle.

[0035]FIG. 7 is a ladder diagram that depicts a periodic attempt torestore the ADI/R operation during sustained DDD/R operation.

[0036]FIG. 8 is a ladder diagram of the pacing operation in the eventthat the patient develops an atrial tachycardia.

DETAILED DESCRIPTION OF THE DRAWINGS

[0037]FIG. 1 is an illustration of an implantable medical device systemadapted for use in accordance with the present invention. The medicaldevice system shown in FIG. 1 includes an implantable device 10—apacemaker in this embodiment—that has been implanted in a patient 12. Inaccordance with conventional practice in the art, pacemaker 10 is housedwithin a hermetically sealed, biologically inert outer casing, which mayitself be conductive so as to serve as an indifferent electrode in thepacemaker's pacing/sensing circuit. One or more pacemaker leads,collectively identified with reference numeral 14 in FIG. 1 areelectrically coupled to pacemaker 10 in a conventional manner and extendinto the patient's heart 16 via a vein 18. Disposed generally near thedistal end of leads 14 are one or more exposed conductive electrodes forreceiving electrical cardiac signals and/or for delivering electricalpacing stimuli to heart 16. As will be appreciated by those of ordinaryskill in the art, leads 14 may be implanted with its distal end situatedin the atrium and/or ventricle of heart 16.

[0038] Although the present invention will be described herein in oneembodiment which includes a pacemaker, those of ordinary skill in theart having the benefit of the present disclosure will appreciate thatthe present invention may be advantageously practiced in connection withnumerous other types of implantable medical device systems, and indeedin any application in which it is desirable to provide the preferredADI/R pacing mode, as may occur in ICDs.

[0039] Also depicted in FIG. 1 is an external programming unit 20 fornon-invasive communication with implanted device 10 via uplink anddownlink communication channels, to be hereinafter described in furtherdetail. Associated with programming unit 20 is a programming head 22, inaccordance with conventional medical device programming systems, forfacilitating two-way communication between implanted device 10 andprogrammer 20. In many known implantable device systems, a programminghead such as that depicted in FIG. 1 is positioned on the patient's bodyover the implant site of the device (usually within 2- to 3-inches ofskin contact), such that one or more antennae within the head can sendRF signals to, and receive RF signals from, an antenna disposed withinthe hermetic enclosure of the implanted device or disposed within theconnector block of the device, in accordance with common practice in theart.

[0040]FIG. 2 is a perspective view of programming unit 20 in accordancewith the presently disclosed invention. Internally, programmer 20includes a processing unit (not shown in the Figure) that in accordancewith the presently disclosed invention is a personal computer typemotherboard, e.g., a computer motherboard including an Intel Pentium 3microprocessor and related circuitry such as digital memory. The detailsof design and operation of the programmer's computer system will not beset forth in detail in the present disclosure, as it is believed thatsuch details are well-known to those of ordinary skill in the art.

[0041] Referring to FIG. 2, programmer 20 comprises an outer housing 60,which is preferably made of thermal plastic or another suitably ruggedyet relatively lightweight material. A carrying handle, designatedgenerally as 62 in FIG. 2, is integrally formed into the front ofhousing 60. With handle 62, programmer 20 can be carried like abriefcase.

[0042] An articulating display screen 64 is disposed on the uppersurface of housing 60. Display screen 64 folds down into a closedposition (not shown) when programmer 20 is not in use, thereby reducingthe size of programmer 20 and protecting the display surface of display64 during transportation and storage thereof.

[0043] A floppy disk drive is disposed within housing 60 and isaccessible via a disk insertion slot (not shown). A hard disk drive isalso disposed within housing 60, and it is contemplated that a hard diskdrive activity indicator, (e.g., an LED, not shown) could be provided togive a visible indication of hard disk activation.

[0044] As would be appreciated by those of ordinary skill in the art, itis often desirable to provide a means for determining the status of thepatient's conduction system. To accomplish this task and providesuitable ECG tracings, programmer 20 is equipped with external ECG leads24.

[0045] In accordance with the present invention, programmer 20 isequipped with an internal printer (not shown) so that a hard copy of apatient's ECG or of graphics displayed on the programmer's displayscreen 64 can be generated. Several types of printers, such as theAR-100 printer available from General Scanning Co., are known andcommercially available.

[0046] In the perspective view of FIG. 2, programmer 20 is shown witharticulating display screen 64 having been lifted up into one of aplurality of possible open positions such that the display area thereofis visible to a user situated in front of programmer 20. Articulatingdisplay screen is preferably of the LCD or electro-luminescent type,characterized by being relatively thin as compared, for example, acathode ray tube (CRT) or the like.

[0047] As would be appreciated by those of ordinary skill in the art,display screen 64 is operatively coupled to the computer circuitrydisposed within housing 60 and is adapted to provide a visual display ofgraphics and/or data under control of the internal computer.

[0048] Programmer 20 described herein with reference to FIG. 2 isdescribed in more detail in U.S. Pat. No. 5,345,362 issued to Thomas J.Winkler, entitled “Portable Computer Apparatus With Articulating DisplayPanel,” which patent is hereby incorporated herein by reference in itsentirety. The Medtronic Model 9790 programmer is the implantabledevice-programming unit with which the present invention may beadvantageously practiced.

[0049]FIG. 3 is a block diagram of the electronic circuitry that makesup pulse generator 10 in accordance with the presently disclosedinvention. As can be seen from FIG. 3, pacemaker 10 comprises a primarystimulation control circuit 20 for controlling the device's pacing andsensing functions. The circuitry associated with stimulation controlcircuit 20 may be of conventional design, in accordance, for example,with what is disclosed U.S. Pat. No. 5,052,388 issued to Sivula et al.,“Method and apparatus for implementing activity sensing in a pulsegenerator.” To the extent that certain components of pulse generator 10are conventional in their design and operation, such components will notbe described herein in detail, as it is believed that design andimplementation of such components would be a matter of routine to thoseof ordinary skill in the art. For example, stimulation control circuit20 in FIG. 3 includes sense amplifier circuitry 24, stimulating pulseoutput circuitry 26, a crystal clock 28, a random-access memory andread-only memory (RAM/ROM) unit 30, and a central processing unit (CPU)32, all of which are well-known in the art.

[0050] Pacemaker 10 also includes internal communication circuit 34 sothat it is capable communicating with external programmer/control unit20, as described in FIG. 2 in greater detail.

[0051] With continued reference to FIG. 3, pulse generator 10 is coupledto one or more leads 14 which, when implanted, extend transvenouslybetween the implant site of pulse generator 10 and the patient's heart16, as previously noted with reference to FIG. 1. Physically, theconnections between leads 14 and the various internal components ofpulse generator 10 are facilitated by means of a conventional connectorblock assembly 11, shown in FIG. 1. Electrically, the coupling of theconductors of leads and internal electrical components of pulsegenerator 10 may be facilitated by means of a lead interface circuit 19which functions, in a multiplexer-like manner, to selectively anddynamically establish necessary connections between various conductorsin leads 14, including, for example, atrial tip and ring electrodeconductors ATIP and ARING and ventricular tip and ring electrodeconductors VTIP and VRING, and individual electrical components of pulsegenerator 10, as would be familiar to those of ordinary skill in theart. For the sake of clarity, the specific connections between leads 14and the various components of pulse generator 10 are not shown in FIG.3, although it will be clear to those of ordinary skill in the art that,for example, leads 14 will necessarily be coupled, either directly orindirectly, to sense amplifier circuitry 24 and stimulating pulse outputcircuit 26, in accordance with common practice, such that cardiacelectrical signals may be conveyed to sensing circuitry 24, and suchthat stimulating pulses may be delivered to cardiac tissue, via leads14. Also not shown in FIG. 3 is the protection circuitry commonlyincluded in implanted devices to protect, for example, the sensingcircuitry of the device from high voltage stimulating pulses.

[0052] As previously noted, stimulation control circuit 20 includescentral processing unit 32 which may be an off-the-shelf programmablemicroprocessor or micro controller, but in the present invention is acustom integrated circuit. Although specific connections between CPU 32and other components of stimulation control circuit 20 are not shown inFIG. 3, it will be apparent to those of ordinary skill in the art thatCPU 32 functions to control the timed operation of stimulating pulseoutput circuit 26 and sense amplifier circuit 24 under control ofprogramming stored in RAM/ROM unit 30. It is believed that those ofordinary skill in the art will be familiar with such an operativearrangement.

[0053] With continued reference to FIG. 3, crystal oscillator circuit28, in the presently preferred embodiment a 32,768-Hz crystal controlledoscillator provides main timing clock signals to stimulation controlcircuit 20. Again, the lines over which such clocking signals areprovided to the various timed components of pulse generator 10 (e.g.,microprocessor 32) are omitted from FIG. 3 for the sake of clarity.

[0054] It is to be understood that the various components of pulsegenerator 10 depicted in FIG. 3 are powered by means of a battery (notshown) that is contained within the hermetic enclosure of pacemaker 10,in accordance with common practice in the art. For the sake of clarityin the Figures, the battery and the connections between it and the othercomponents of pulse generator 10 are not shown.

[0055] Stimulating pulse output circuit 26, which functions to generatecardiac stimuli under control of signals issued by CPU 32, may be, forexample, of the type disclosed in U.S. Pat. No. 4,476,868 to Thompson,entitled “Body Stimulator Output Circuit,” which patent is herebyincorporated by reference herein in its entirety. Again, however, it isbelieved that those of ordinary skill in the art could select from amongmany various types of prior art pacing output circuits that would besuitable for the purposes of practicing the present invention.

[0056] Sense amplifier circuit 24, which is of conventional design,functions to receive electrical cardiac signals from leads 14 and toprocess such signals to derive event signals reflecting the occurrenceof specific cardiac electrical events, including atrial contractions(P-waves) and ventricular contractions (R-waves). CPU provides theseevent-indicating signals to CPU 32 for use in controlling thesynchronous stimulating operations of pulse generator 10 in accordancewith common practice in the art. In addition, these event-indicatingsignals may be communicated, via uplink transmission, to externalprogramming unit 20 for visual display to a physician or clinician.

[0057] Those of ordinary skill in the art will appreciate that pacemaker10 may include numerous other components and subsystems, for example,activity sensors and associated circuitry. The presence or absence ofsuch additional components in pacemaker 10, however, is not believed tobe pertinent to the present invention, which relates primarily to theimplementation and operation of communication subsystem 34 in pacemaker10, and an associated communication subsystem in external unit 20.

[0058]FIG. 4 is a ladder diagram of the ADI/R operation, specifically aMarker Channel® Diagram. With the help of the NBG Code, one familiarwith the state of the art will be able to discern that the letter in thefirst position (A) means that the pacemaker (or other implanted device)will pace the atrium in the absence of an atrial sensed event. Thesecond letter (D) implies that the pacemaker will sense in dualchambers, that is, both the atrial and ventricular chambers. The thirdletter (I) means that, upon sensing in either chamber, pacing will beinhibited in that specific chamber. The final letter, R, implies thatthe device may be rate responsive, that is, altering the atrial rate inresponse to an artificial sensor, such as a Piezo-electrical crystal,accelerometer, minute ventilation, etc.

[0059] The operation of the preferred ADI/R mode is depicted in theladder diagram as follows. Atrial paced (or sensed) event 1 initiates anon-programmable, auto-adjusting (e.g., 100-150 millisecond) blankingperiod 4, followed by auto-adjusting atrial sensitivity (not shown).Sensing circuitry (see FIG. 3) determines if and when ventricular sensedevent 2 has occurred. If detected, timing circuitry (see FIG. 3)initiates VA interval 9. Other timing, blanking periods, and refractoryperiods serve the following purposes. A programmable ventricularblanking period 8 prevents sensing of atrial pace 1 on the ventricularchannel, sometimes termed “crosstalk.” Ventricular sensed event 2 startsa 120 millisecond post ventricular atrial blanking (PVAB) period 6,followed by auto-adjusting atrial sensitivity. PVAB 6 serves the purposeof preventing sensing of the R-wave or T-wave on the atrial channel,termed “far-field R-wave sensing.” Ventricular sensed event 2 alsostarts 100 millisecond ventricular blanking 7 followed by auto-adjustingventricular sensitivity. This period serves the purpose of preventingsensing of the ventricular output pulse or the ventriculardepolarization itself. Repolarization, or T-wave 3, follows R-wave 2.Ventricular event 2 detected by sensing circuitry (see FIG. 3) sendssignal to timing circuitry to start VA interval 9, leading to the nextatrial pacing cycle.

[0060] Taking into account that this mode would be used primarily withSick Sinus patients who have full or some degree of intact AVconduction, this type of operation as depicted for the ADI/R mode issomething the clinician or physician would expect to occur. In thepresence of intact AV conduction, even if it is prolonged, the pacemakerwill maintain the ADI/R operation/mode. Sensed ventricular events wouldoccur in the vast majority of cardiac cycles (that is, PQRST). FIG. 5teaches what will occur should the patient develop transient AV blockfor one or a few cardiac cycles.

[0061]FIG. 5 is a ladder diagram of the committed DDI/R operation in theevent that the patient develops transient AV block. The purpose of thecommitted DDI/R operation is to maintain ventricular support in thepresence of AV block. Briefly stated, the implanted device mode switchesfrom the preferred ADI/R to the committed DDI/R for one cycle.

[0062] The timing of the Committed DDI/R is as follows. In the DDI/Rmode (third pacing cycle, labeled DDI/R), AV interval 5 is set to ashort 80 milliseconds, following the Paced P-wave due to the absence ofany sensed ventricular event between the second and third atrial pacedevents. The purpose of this short AV interval 5 is to suppresscompetition between ventricular pacing pulse culminating in paced R-wave13 and any potential intrinsic R-wave with a delayed conduction from theprevious paced atrial event. Assuming the presence of such an intrinsicR-wave, the timing of the ventricular output pulse would normally resultin a ventricular pacing pulse falling into the absolute refractoryperiod of the intrinsic, conducted R-wave, resulting in a psuedo-fusionbeat (not shown). This operation is intended to prevent the onset of aventricular tachycardia, should the ventricular pacing pulse fall intothe relative refractory period of the ventricle, commonly called “pacingon T” phenomenon.

[0063] Continuing with the timing in FIG. 5, paced R-wave 13 starts a120 millisecond ventricular blanking period 7, followed by autoadjusting ventricular sensitivity (not shown). Paced R-wave 13 alsostarts a 120 millisecond PVAB 6 followed by auto adjusting atrialsensitivity (not shown). Assuming the transient AV block self-correctsand a sensed R-wave is detected, the preferred ADI/R resumes with thenext paced or sensed P-wave, as is depicted in FIG. 4.

[0064]FIG. 6 is a ladder diagram that depicts the pacing operation inthe event that the patient develops AV block that persists for more thanone cycle. This figure is a continuation of FIG. 5. Following theone-cycle mode switch to DDI/R, VA interval 9 times out, resulting inatrial paced event 1. A very long (e.g. 400 millisecond or up to 65% ofthe sensor-indicated AV interval) 17 is used in an attempt to promote AVconduction. If, however, AV interval 17 is not interrupted by a sensed,intrinsic R-wave, as is depicted in the first cycle (labeled ADI/R), thepacemaker immediately switches to the DDD/R mode. In the event that asensed, intrinsic R-wave does occur, the device would revert to theADI/R operation (not shown). The DDD/R operation, with the programmed AVinterval, will be sustained until a sensed, intrinsic R-wave isdetected, as is shown in FIG. 7. Periodic attempts to force restorationof the ADI/R operation are performed (as depicted in FIG. 7). Modeswitching to the DDI/R mode will occur in the event that an atrialtachycardia is detected (see FIG. 8).

[0065]FIG. 7 is a ladder diagram that depicts a periodic attempt torestore the ADI/R operation during sustained DDD/R operation. Asmentioned, the DDD/R mode may become the sustained mode of operation inthe event that the patient develops a prolonged AV block, such as mightoccur with rate-dependent AV block. In such cases, the device may beprogrammed to revert to ADI/R 1 after a programmable number of DDD/Rcycles. Then, the device looks for a ventricular sensed event, e.g., at23 following atrial pace 1. In the event that a sensed, intrinsic R-waveis detected, the ADIR operation is immediately resumed. In the absenceof a ventricular sensed event, the device continues to operate in theDDD/R mode, as indicated in the third cycle of FIG. 7.

[0066]FIG. 8 is a ladder diagram of the pacing operation in the eventthat the patient develops an atrial tachycardia. A sick sinus patientoften has episodes of atrial tachycardia, atrial flutter, or atrialfibrillation. During these episodes, the pacing operation must be suchthat the ventricular pacing rate will neither be synchronized to thefast atrial rate nor so slow as to cause symptoms.

[0067] In FIG. 5 it was noted that the device, while operating in theADI/R mode, can switch to the DDI/R mode. The DDI/R mode is well suitedfor pacing in the presence of an atrial tachycardia because it will notallow ventricular synchronization to a fast atrial rate nor will itallow the ventricular pacing rate to go below the programmed lower rate.Therefore, when an atrial tachycardia does occur, as shown in FIG. 8,fast atrial sensed events 27 without a conducted ventricular event haveno effect on ventricular timing 9. Since there is no ventricular event,the operation immediately switches to the DDI/R mode. In the presence ofan atrial tachycardia, the V-V interval 9 times out so that paced R-wave8 will occur at the faster of the programmed lower rate orsensor-indicated rate in the DDI/R mode. The operation depicted in FIG.8 will continue so long as the atrial tachycardia persists. Upontermination of the atrial tachycardia, the preferred ADI/R will resumeas shown in FIGS. 4 or 7, depending on how the heart recovers from theatrial tachyarrhythmia. If the atrial tachyarrhythmia terminatesabruptly, the prompt restoration of the ADI/R mode takes place (see FIG.4). If, however, the atrial tachyarrhythmia “cools down” slowly, theremay be a period of DDD/R pacing with periodic attempts to restore ADI/Rpacing as shown in FIG. 7.

[0068] Since all patients may experience premature ventricular events(PVCs), the ADI/R operation treats the PVC as a sensed ventricular eventthat resets the VA interval. Such PVCs can occur during normal sinusrhythm or during an atrial tachyarrhythmia. In either case, the A-Atiming is interrupted and reset by the PVC (not shown), an operationwell known to those familiar with the state of the art. A PVC sensedduring an atrial tachycardia inhibits the scheduled ventricular pacingpulse and resets the ventricular lower rate and sensor-indicatedventricular rate timers.

[0069] It is to be understood that the above description is intended tobe illustrative and, not restrictive. Many other embodiments will beapparent to those of skill in the art upon reading and understanding theabove description. The scope of the invention should, therefore, bedetermined with reference to the appended claims, along with the fullscope of equivalents to which such claims are entitled.

What is claimed is:
 1. A system for implementing a preferential ADI/R mode in a pacing regimen, the system comprising: means for implementing an ADI/R pacing mode; means for detecting AV conduction on a beat to beat basis; means for sensing an AV block; means for automatically switching to a DDD/R mode; means for ensuring a paced R-wave; and means for resuming the ADI/R pacing mode.
 2. The system of claim 1 wherein said means for implementing the ADI/R mode includes an on/off selection structure operating as a subset of a programmed DDB/R mode.
 3. The system of claim 1 wherein said means for detecting AV conduction includes a sensing circuit.
 4. The system of claim 1 wherein said means for sensing an AV block includes means for switching from ADI/R to DDI/R for a cycle.
 5. The system of claim 1 wherein said means for switching to a DDD/R mode includes mean s for interrupting an AV interval using a sensed intrinsic R-wave.
 6. The system of claim 1 wherein said means for ensuring a paced R-wave means for sustaining the DDD/R operation until a sensed intrinsic R-wave is detected.
 7. The system of claim 1 wherein said means for resuming the ADI/R mode includes means for periodically attempting to force the ADI/R mode.
 8. A pacing operation of a software system implemented in a medical device to program pacing cycles based on a preferred ADI/R mode, the system comprising: implementing an ADI/R pacing mode; detecting AV conduction on a beat to beat basis; sensing an AV block; switching to a DDD/R mode automatically when said AV block is sensed; ensuring a paced R-wave; and resuming ADI/R pacing mode when said paced R-wave is ensured. 